What are antibodies and are they proteins in the context of a patient with recurrent infections and immune deficiency?

What Are Antibodies and Are They Proteins?

Yes, antibodies (also called immunoglobulins) are proteins—specifically, heavy plasma proteins with sugar chains added through glycosylation—that function as the primary effector molecules of humoral immunity. 1

Basic Structure and Function

Antibodies are complex protein molecules produced by B cells that serve multiple critical immune functions 1:

• Neutralization of pathogens and toxins 1
• Agglutination (clumping) of foreign particles 1
• Complement fixation and activation 1
• Activation of effector cells 1

The Five Classes of Immunoglobulins

The immune system produces five distinct types of antibody proteins, each with specialized roles 2:

• IgG (with four subclasses: IgG1, IgG2, IgG3, IgG4) - the most abundant antibody in serum 2
• IgM - the first antibody produced in response to infection 2
• IgA - critical for mucosal immunity, especially in the gastrointestinal tract 2
• IgD - involved in B cell activation 2
• IgE - mediates allergic responses and defense against parasites 2

Clinical Relevance in Antibody Deficiency

In the context of recurrent infections and immune deficiency, understanding antibodies as proteins is essential because 3:

• Antibody deficiency disorders account for approximately half of all primary immunodeficiency diagnoses 3
• These conditions result from impaired antibody protein production due to molecular defects in B cells or failed B-cell/T-cell interaction 4
• The characteristic clinical presentation is recurrent sinopulmonary infections with encapsulated bacteria and viral respiratory/gastrointestinal infections 3

Diagnostic Considerations

When evaluating suspected antibody deficiency, the protein nature of antibodies allows measurement through 3:

• Quantitative immunoglobulin levels (measuring antibody protein concentration) 3
• Functional antibody responses to protein antigens (diphtheria, tetanus) and polysaccharide antigens (pneumococcal vaccine) 3

A critical pitfall: Simply measuring antibody protein concentration is insufficient—functional antibody assessment (such as opsonophagocytic assays measuring actual killing of organisms) provides more clinically meaningful information than quantity alone 3

Treatment Implications

Because antibodies are proteins, patients with severe antibody deficiency can receive immunoglobulin replacement therapy—essentially passive administration of antibody proteins from pooled donor plasma 5:

• This lifelong protein replacement therapy prevents the majority of infections when given regularly 5
• The therapy works by providing the missing antibody proteins that the patient's immune system cannot produce 5